The invention addresses the problem of the removal of carbon dioxide from its mixtures with methane such as acid gas. Acid gas is natural gas containing acidic gas components such as carbon dioxide (CO2) and hydrogen sulphide (H2S).
About 40% of the world natural gas reservoirs contain significant concentrations of carbon dioxide and hydrogen sulphide. Such acid gas fields are found in Europe, Africa, North and South America and the Far East; the Middle East and Central Asia hold the largest volumes. In order to efficiently use the hydrocarbons from acid gas, the acidic components CO2 and H2S need to be removed. In the last years the efforts for the development of new technologies that can make the exploitation of these reserves economically convenient are increased. At the same time, international and local environmental regulations push towards an increasing limitation of the CO2 emissions to the atmosphere.
For the exploitation of an acid gas field, the key economic driver is the cost to separate and recover the acid gas components (CO2, H2S). The traditional chemical separation of CO2 and H2S with amines is not competitive when the stream contains high quantity of acidic components because this involves high costs proportional to the quantity of the components to be removed. Another problem is that in the known separation processes components CO2 and H2S are separated as low pressure vapours, therefore leading to high recompression costs for re-injection.
In the prior art cryogenic processes are known and used for the separation of carbon dioxide from methane in an acid gas. A problem during cryogenic distillation is however the build-up of solid carbon dioxide inside the distillation column. In the prior art, two cryogenic processes are known and currently used in industrial applications wherein the build-up is avoided. These are the Ryan Holmes process described in U.S. Pat. No. 4,318,723 and the CFZ (Controlled Freeze Zone) process described in U.S. Pat. No. 4,533,372.
In the Ryan Holmes process the build-up of solid carbon dioxide is addressed by adding to the natural gas a third component (usually a C3-C6 hydrocarbon) that prevents the carbon dioxide solidification inside the distillation column. Therefore this added component has to be separated from the bottom product of the cryogenic distillation in a regeneration unit. The addition of a third component (to be added and separated) leads to an increase of investment costs (CAPEX) and operating costs (OPEX).
The CFZ process is based on a complex cryogenic distillation column. Carbon dioxide solidification is allowed in a specific zone of the column, the controlled freeze zone, where the deposited solid CO2 has to be molten and extracted. The process requires an ad-hoc designed column. The use of a complex solid-liquid separation column, with an ad-hoc design, leads to an increase of CAPEX and OPEX costs.
Other processes are described, for example, in U.S. Pat. No. 4,284,423 which is a process for the separation of CO2 from methane by distillation wherein the gas mixture is fed first to a low pressure column and subsequently to a high pressure column. The stream from the low pressure column needs to be compressed and cooled before entering the high pressure column. After reproduction of the exemplified processes with commercial process simulators, it is found that this process leads to the formation of solid CO2.
DE2622662 discloses a distillation process for the separation of CO2 from methane wherein the top product from a high-pressure distillation column is cooled and completely condensed and subsequently fed to a second column operating at a lower pressure. Reproduction of the exemplified processes with commercial process simulators shows that the simulation of this process does not reach convergence.
It is therefore desirable to provide a process for the removal of CO2 from acid gas which would not have the above-mentioned drawbacks. In particular, the process should achieve complete separation of methane also at a high level of acidic components, avoid solid CO2 precipitation and moreover be economical.